Promise.resolve()

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.

Promise.resolve(value) 方法回傳一個以 value 判定結果的 Promise 物件。若 value 是個 thenable（例如，具有 then() 方法，則回傳的 promise 將依其結果採取其最終狀態；若 value 是 promise，則作為呼叫 Promise.resolve 之結果；其他情形都將回傳以 value 實現的 promise。

語法

Promise.resolve(value);
Promise.resolve(promise);
Promise.resolve(thenable);

參數

value: 將被 Promise 實現的引數（argument）。可以是個 Promise 或待解決的 thenable。

回傳值

以 value 或作為 value 的 promise 解決的 Promise。

描述

靜態函式 Promise.resolve 回傳判定後的 Promise。

範例

使用 `Promise.resolve` 靜態方法

Promise.resolve("Success").then(
  function (value) {
    console.log(value); // "Success"
  },
  function (value) {
    // not called
  },
);

判定陣列

var p = Promise.resolve([1, 2, 3]);
p.then(function (v) {
  console.log(v[0]); // 1
});

判定另一個 `Promise`

var original = Promise.resolve(33);
var cast = Promise.resolve(original);
cast.then(function (value) {
  console.log("value: " + value);
});
console.log("original === cast ? " + (original === cast));

// logs, in order:
// original === cast ? true
// value: 33

由於 handler 是非同步地被調用而導致相反的紀錄順序。經由這篇文章了解 then 如何運作。

判定 thenable 及拋出 Error

// Resolving a thenable object
var p1 = Promise.resolve({
  then: function (onFulfill, onReject) {
    onFulfill("fulfilled!");
  },
});
console.log(p1 instanceof Promise); // true, object casted to a Promise

p1.then(
  function (v) {
    console.log(v); // "fulfilled!"
  },
  function (e) {
    // not called
  },
);

// Thenable throws before callback
// Promise rejects
var thenable = {
  then: function (resolve) {
    throw new TypeError("Throwing");
    resolve("Resolving");
  },
};

var p2 = Promise.resolve(thenable);
p2.then(
  function (v) {
    // not called
  },
  function (e) {
    console.log(e); // TypeError: Throwing
  },
);

// Thenable throws after callback
// Promise resolves
var thenable = {
  then: function (resolve) {
    resolve("Resolving");
    throw new TypeError("Throwing");
  },
};

var p3 = Promise.resolve(thenable);
p3.then(
  function (v) {
    console.log(v); // "Resolving"
  },
  function (e) {
    // not called
  },
);

規範

Specification
ECMAScript Language Specification # sec-promise.resolve

瀏覽器相容性

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參見

Promise